Gases dissolved in liquids always pose the risk of increased corrosion in the plant. Gaseous impurities are present in liquid sulphur in particular in the form of H2S (hydrogen sulphide), H2Sx (hydrogen polysulphides), SO2 (sulphur dioxide) and possibly other gaseous sulphur compounds.
H2S is a very dangerous compound, since it is poisonous in the air in a concentration of less than 10 ppm by volume, and is life-threatening at a concentration of some 100 ppm by volume. In addition, hydrogen sulphide may form an explosive mixture in air upon reaching a concentration of >3.4% by volume.
H2S tends to outgas from the liquid sulphur when this is shaken and cooled, which occurs in particular during handling, storage and transport. The dangerous gas then collects in the gaseous phase of the storage and transport containers. If solid sulphur is produced from non-degassed liquid sulphur, H2S and SO2 also evolve naturally from the sulphur. In both cases, non-degassed liquid sulphur is thus a source of volatile emissions of H2S and SO2 in storage areas and thus causes not only contaminant nuisances and environmental pollution, but also poses a considerable safety risk.
For all these reasons, it is necessary to remove H2S by degassing from the raw sulphur produced so as to prevent all risks of toxicity, development of fire and risk of explosion during handling, storage and transport.
The industrial companies concerned with sulphur (producers, hauliers and end consumers) have agreed on international regulations which define the specification of the commercial product, and inter alia have limited the total residual amount of hydrogen sulphide to a maximum of 10 ppm by weight.
A range of methods are known with which the content of hydrogen sulphide in the liquid sulphur is to be reduced to less than 10 ppm.
With the D'GAASS method, as is described in WO 95/07854 A for example, sulphur is fed in a column having a plurality of separation stages in counterflow to pressurised air. The phase transport is facilitated by column internals to increase the contact area.
In the Shell degassing process (for example see U.S. Pat. No. 6,149,887), the phase transition from the liquid into the gas phase is facilitated by bubbling air into the sulphur. By air flushing, the H2S is fed together with the bubbled-in air for afterburning. The Exxon Mobil degassing technology (for example see U.S. Pat. No. 7,927,577 B2) also functions in a similar manner. In this case, too, a multiplicity of Venturi nozzles are located on the base of a container which is flooded, at least in part, with liquid sulphur and through which the stripping gas introduced into the nozzles forms small bubbles and thus removes dissolved hydrogen sulphide from the liquid sulphur.
Further methods which are based on a degassing of the liquid sulphur using a stripping gas are described, for example, in U.S. Pat. No. 6,149,887 or WO 03/106335 A1. In U.S. Pat. No. 6,149,887 a gas is fed through the liquid sulphur, wherein the liquid sulphur itself is also pumped around. It passes through at least two degassing compartments, wherein these degassing compartments are each divided again into two sub-compartments and the degassing compartments are separated from one another by at least one partition wall. The gas is then injected into at least one of the sub-compartments in finely distributed form via the base. A flow of the liquid sulphur is produced by openings between sub-compartments and the degassing compartments, whereby the transition conditions at the gas-liquid interface are favourable.
WO 03/106335 A1 lastly describes a method for removing hydrogen sulphides from liquid sulphur, in which liquid sulphur is introduced into a container from top to bottom and flows into an outer ring via an outlet in the lower face of the device, which outer ring is gassed with air.
The common point of all these methods is that no additional degassing agent can be introduced into the liquid sulphur. With suitable procedure, the hydrogen sulphide content can thus still be reduced below the legally required threshold of 10 ppm by volume, wherein air has to be used, however, as stripping gas.
In addition to desired reactions in the system, the use of air also leads to a direct oxidation of H2S and H2Sx as well as of the sulphur itself, whereby some SO2 is formed in the sulphur in a dissolved state in an amount of up to several 100 ppm by weight, depending on the temperature.
Some of the SO2 is again found in the stripping air together with the outgassed H2S, which contributes to the sulphur losses of the method as a whole.
Owing to the introduction of the oxygen contained in the air as well as the formation of sulphur dioxide, further oxidation reactions also take place. As a result thereof, the fraction of SO3 and other high-grade oxidation products contained in the system, namely sulphur in an oxidation stage of 6+(H2SO4, polythionic acids, etc.) is considerably increased, which in turn leads to an increase in the acidity of the sulphur and subsequent corrosivity produced thereby.
The described methods for degassing generally require very long times of degassing (10 to 20 hours), which leads to increased SO2 formation.
If, by contrast, inert gas is used as stripping gas, all sulphur compounds cannot be removed reliably. These may decompose with the further use of liquid sulphur, whereby new H2S is formed. A degassing, which thus removes the hydrogen sulphide only in the short term, and not the hydrogen polysulphides, is therefore not suitable for processing liquid sulphur in such a way that the legal thresholds of residual H2S are observed in a sustainable manner.
For this reason, a range of methods for degassing liquid sulphur have been developed which utilise a catalyst. Above all, the Aquisulf® method is known (see EP 0 252 836 B1). In this case polysulphide chains are destroyed catalytically, wherein the Aquisulf® liquid catalyst is used. The H2S thus produced and the dissolved H2S are transported from the liquid phase into the gaseous phase by atomising the sulphur into chambers by means of pumps and spraying nozzles.
DE 28 42 141 also describes a method for degassing liquid sulphur using a catalyst. A three-stage column is flushed with an ammonia-containing nitrogen gas and the individual stages are charged with sulphur and a gas mixture in co-current flow.
However, the use of ammonia leads to lasting damage to the plant components.